Ch. XXII.] 
AMOUNT OF DENUDATION. 
313 
of large districts of the chalk and other denuded secondary 
rocks. But these facts do not, we think, militate against the 
theory above proposed, for we have assumed a long-continued 
series of elevatory movements in a region where the degrada- 
tion and reproduction of strata were in progress. 
If this be granted, it is evident that the great antagonist 
powers, the igneous and the aqueous, would, throughout the 
whole period, be brought into play in their fullest energy, the 
igneous labouring continually to produce the greatest inequality 
of surface, by uplifting certain lines of country and depressing 
others ; the aqueous no less incessantly engaged in reducing the 
whole to a level, by cutting off the summits of the upraised 
tracts, and throwing the materials thence removed into the 
adjoining hollows. If the volcanic forces eventually prevail, 
so as to convert the whole region into land, we must expect 
that some of the materials drifted into the hollows, and formino- 
the newer strata, will be brought up to view, while the de- 
nuded districts are raised at the same time. If these last 
continue, in general, to occupy a higher position above the 
level of the sea, it is all that can be expected after the levelling 
operations before alluded to. 
Now the tracts occupied by our Eocene formations are low, 
not so much with reference to the secondary rocks which re- 
main, as to these masses which must be supposed by our theory 
to have disappeared, having been carried away by denudation. 
Let the portions removed from the space intervening between 
the North and South Downs, and which are expressed by faint 
lines in our section, wood-cut No. 63, be restored, and we may 
readily conceive that those masses may have formed shoals 
and dry land for ages before any part of our tertiary basins 
emerged. 
The estimate of Mr. Martin is not, perhaps, exaggerated, 
when he computes the probable thickness of strata removed 
from the highest part of the Forest ridge to be about 1900 
feet. So that if we restore to Crowborough Hill, in Sussex, 
the beds of Weald clay, Lower green-sand, Gault, and chalk, 
